There are a number of greatly different laserbeam applications wherein it is a requirement to expand, or compress, the cross sectional dimension of the beam within, or before entering, a given system. The Bragg-diffraction acousto-optic laserbeam scanner represents one such application which can be kept in mind in connection with the discussion herein of the present improvement in laser light beam expanders and compressors. In the case of the scanner, ultrasonic power needs may be kept within reasonable limits when the beam is expanded so as to provide an elliptical section whose major axis is located in the deflection plane.
A normally circular beam of light can, of course, be expanded in two dimensions with a spherical telescope and in but a single dimension by either a cylindrical lens system or a prismatic beam expander. The latter is preferred over the cylindrical lens system in providing an elliptical shaped beam. Experience has shown that the prismatic method has a light efficiency greatly exceeding that of the cylindrical lens and without introducing the least aberation.
Prismatic beam expansion is certainly not new, however. It was in 1810 that David Brewster first observed light beam expansion with the use of optical prisms. The angle of incidence of a beam polarized parallel to incident plane, when maximum magnification and minimum light reflection at a prism surface is realized, is known as Brewster's angle and at which angle a magnification of a beam of light can be shown to be equal to the index of refraction of the prism material.